Hesperosaurus (from Ancient Greek ἕσπερος (hésperos), meaning "western", and σαῦρος (saûros), meaning "lizard") is a herbivorous stegosaurian dinosaur from the Kimmeridgian age of the Jurassic period, approximately 156 million years ago. Fossils of Hesperosaurus have been found in the state of Wyoming and Montana in the United States of America since 1985. The type species Hesperosaurus mjosi was named in 2001. It is from an older part of the Morrison Formation, and so a little older than other Morrison stegosaurs. Several relatively complete skeletons of Hesperosaurus are known. One specimen preserves the first known impression of the horn sheath of a stegosaurian back plate. Hesperosaurus was a member of the Stegosauridae, quadrupedal plant-eaters protected by vertical bony plates and spikes. It was closely related to Stegosaurus and was similar to it in having two rows of, possibly alternating, plates on its back and four spikes on its tail end. The plates on its back were perhaps not as tall, but were longer. It possibly had a deeper skull than Stegosaurus.
Discovery and species
In 1985, fossil hunter Patrick McSherry, at the ranch of S.B. Smith in Johnson County, Wyoming, found the remains of a stegosaur. As he had difficulty securing the specimen due to the hard rock matrix, he sought help from Ronald G. Mjos and Jeff Parker of Western Paleontological Laboratories, Inc. They, in turn, cooperated with paleontologist Dee Hall of Brigham Young University. At first, it was assumed it represented an exemplar of Stegosaurus. However, Clifford Miles, while preparing the remains, recognised that they belonged to a species new to science. The type species Hesperosaurus mjosi was named and described in 2001 by Kenneth Carpenter, Clifford Miles, and Karen Cloward. The generic name is derived from the Greek ἕσπερος, hesperos, "western", in reference to its location in the western United States. The specific name honours Mjos who, apart from his involvement in the process of collecting and preparing the holotype, also had a cast of it made, exhibited with the inventory number DMNH 29431 in the Denver Museum of Natural History. The holotype, HMNH 001 (later HMNS 14), was found in the Windy Hill Member, stratigraphic zone 1 of the lower Morrison Formation, dating from the early Kimmeridgian, about 156 million years old. In 2001, it represented the oldest known American stegosaur. It consists of a nearly complete skull and much of the skeleton. It includes the disarticulated elements of the skull, the rear lower jaws, a hyoid, thirteen neck vertebrae, thirteen back vertebrae, three sacrals, forty-four tail vertebrae, neck ribs, dorsal ribs, chevrons, a left shoulderblade, a complete pelvis, ossified tendons and ten neck and back plates. The skeleton was partly articulated and, in view of healed fractures, belongs to an old individual. It was obtained by the Japanese Hayashibara Museum of Natural Sciences at Okayama. In 2015, the HMNS permanently closed and the holotype was transferred to the Fukui Prefectural Dinosaur Museum, where it was renumbered FPDM-V9674
From 1995 onward at the Howe-Stephens Quarry in Big Horn County, Wyoming, named after the historic location of the Howe Ranch, once explored by Barnum Brown, and the new owner Press Stephens, Swiss palaeontologist Hans-Jakob Siber excavated stegosaur specimens. The first was SMA 3074-FV01 (also SMA M04), a partial skeleton dubbed "Moritz" after Max und Moritz as an earlier Galeamopus sauropod skeleton from the site had been nicknamed "Max". In 1996/97, specimen SMA 0018 (also mistakenly referred to as SMA V03) was uncovered, dubbed "Victoria" after the feeling of victory the exploring team felt when they discovered Allosaurus "Big Al Two" after the original "Big Al" had been confiscated as federal property. It represents a rather complete skeleton with skull, also preserving skin and horn sheath impressions. A third specimen was found in 2002: SMA L02, dubbed "Lilly" after the sisters Nicola and Rabea Lillich assisting the excavations as volunteers. The "Lilly" and "Moritz" specimens are part of the collection of the Aathal Dinosaur Museum in Switzerland; "Victoria" was donated by the Sauriermuseum Aathal, along with "Big Al" and other specimens, to the Natural History Museum of the University of Zurich. At first the three specimens were considered Stegosaurus exemplars. In 2009, initially only "Moritz" and "Lilly" were reclassified as cf. Hesperosaurus mjosi. In 2010, "Victoria" was referred to Hesperosaurus mjosi by Nicolai Christiansen and Emanuel Tschopp. Carpenter had originally concluded that Hesperosaurus was a rather basal stegosaur. However, Susannah Maidment and colleagues in 2008 published a more extensive phylogenetic study in which it was recovered as a derived form, closely related to Stegosaurus and Wuerhosaurus. They proposed that Hesperosaurus should be considered a species of Stegosaurus, with Hesperosaurus mjosi becoming Stegosaurus mjosi; at the same time Wuerhosaurus was renamed into a Stegosaurus homheni. Carpenter, considering the problem more of a philosophical than a scientific nature, in 2010 rejected the synonymy of Hesperosaurus with Stegosaurus stating that in his opinion Hesperosaurus was sufficiently different from Stegosaurus to be named a separate genus. Christiansen in 2010 judged likewise. In 2017, Raven and Maidment recognized both Miragaia and Hesperosaurus as genera distinct from Stegosaurus. In 2015, additional specimens were reported: a concentration of at least five individuals discovered at the JRDI 5ES Quarry near Grass Range, Montana, and two individuals found in the Meilyn Quarry at Como Bluff. In 2018, new specimen of H. mjosi was described from Montana.
Description
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